Swelling kinetics of poly(N-vinylimidazole-co-sodium styrenesulfonate) hydrogels

Hydrogels of N-vinylimidazole and sodium styrenesulfonate were synthesized by radical crosslinking copolymerization. Transient swelling measurements i n water, at room temperature, reveal an unusual behavior. For some gel comp ositions, the swelling time dependency is about a delta function: dry hydro gel pellets swell up readily to reach a high degree of swelling and, sponta neously, they deswell to collapse. Such peculiarity was interpreted in term s of several competing events in the swelling mechanism: water diffusion to ward the gel, chain disentanglement, sodium-proton interchange through the gel-external bath boundary, approaching of chains to allow interaction of s ulfonate groups with neighboring protonated imidazole moieties and diffusio n of water outside the gel. The results of cyclic swelling-deswelling follo wing abrupt changes of composition in the external bath, from water to sodi um hydroxide solution, support that mechanism. (C) 2002 John Wiley & Sons, Inc.